A wide range of industries such as, for example, electric utilities, power plants, oil refineries, off shore oil rigs, gas ethylene companies, chemical plants, coal mining operations, coal prep plants and transfer stations, gas pipelines, plastic manufacturers, granaries, etc. present very hazardous environments in which electrical equipment must be used. Because of these dangerous environments and because of the hazards imposed by the use of electrical equipment in them, various standards have been imposed by the National Electrical Code and by Underwriters Laboratories for the design of electrical equipment for hazardous areas.
The National Electrical Code (NEC) classifies hazardous areas in industrial environments depending upon the properties of the materials found in those environments. (See ANSI/NFPA-70 Article 500.) Class 1 hazardous environments are those containing inflammable gases and vapors. Class 2 hazardous areas are those containing inflammable dusts. Class 3 hazardous areas are those containing fibers and flyinqs. Within each of the aforementioned classes, environments are further grouped in accordance with the particular materials found there. Class 1, Group A, environments are those which include acetylene gas. For example, Class 1, Group A, environments are the most hazardous environments classified by the NEC and these require the most stringent specifications for electrical equipment. Because of the nature of acetylene gas, very little electrical equipment has ever been approved for use in Class 1, Group A, environments.
Class 1, Group B, environments are those including hydrogen gas or manufactured gases containing more than 30% hydrogen by volume. Class 1, Group C, environments are those containing carbon monoxide, hydrogen sulfide, crude oil, etc. Class 1, Group D, environments are those containing acetone, benzine, butyl, ethyl, methyl, propyl and isopropyl alcohols, gasoline, methane, styrene, toluene, vinyl chloride, etc. Telephone equipment has been approved for use in Class 1, Group B, C and D, environments but such equipment is very expensive.
Electrical equipment, including telephones, which is approved for use in Class 1 hazardous areas as classified by the NEC, without the use of approved enclosures, i.e., explosion-proof, purged and pressurized, is referred to as "intrinsically safe". Typical prior art explosion-proof telephones approved for use in Class 1, Groups B, C and D type hazardous environments have enclosures which are not air tight. The enclosures for such explosion-proof telephones, therefore, allow the entry of the surrounding hazardous atmosphere. Any arcing of the device within its enclosure can, therefore, cause ignition inside the enclosure. The enclosure must therefore be constructed to withstand and contain the resulting high pressures caused by the internal explosion. Such enclosures usually include heavy aluminum castings with wide, closely machined cover plates secured in place by many hold down bolts. Any pass through devices through the enclosure, such as push buttons or rotary motion switches, must also maintain certain clearances with a minimum length bushing or sleeve. Such enclosures are heavy and expensive to construct.
It would be desirable to provide a hazardous area telephone for use in Class 1, Group B, C and D, type hazardous environments as classified by the National Electrical Code which did not require the use of an explosion-proof housing of the type described above. It would be further desirable to provide an intrinsically safe telephone for use in Class 1, Group A, environments and it would be even more desirable to produce such a telephone which did not require an explosion-proof housing.
The design of an intrinsically safe telephone which meets Underwriter Laboratories (UL) requirements for installation in hazardous environments of the type classified by the National Electrical Code is not a simple matter.
In addition to meeting the constraints imposed by Underwriters Laboratories, it is also required that an intrinsically safe telephone meet the requirements of the Federal Communications Commission for interface with standard telephone networks. Thus, while an intrinsically safe telephone meeting the object of the present invention must meet the requirements of the NEC and of the Underwriters Laboratories for hazardous environments, it must also have standard line impedances and characteristics in order to interface with preexisting telephone networks.
One type of prior art intrinsically safe telephone is disclosed in U.S. Pat. No. 4,741,031 by Larry P. Grandstaff entitled "Intrinsically Safe Telephone" and assigned to the assignee of the instant application. The contents of this patent are incorporated herein insofar as it is necessary for a full understanding of the present invention. The intrinsically safe telephone of the Grandstaff patent includes a line powered single line instrument, an off-hook circuit for indicating that the single line instrument is off-hook, and a bootstrap power supply for driving the off-hook circuit. The bootstrap power supply draws and stores line power while the single line instrument is in an on-hook condition and uses the stored power to initialize itself in the off-hook condition. The telephone disclosed and claimed in the Grandstaff patent was a significant advance.
Following introduction of the telephone of the Grandstaff patent, however, certain telephone companies have refused to supply service to a single line instrument that draws line power in an on-hook condition. Some telephone companies employ a periodic sweep of their circuits looking for faults. A high resistance short caused by the bootstrap power supply of the Grandstaff patent may appear to the telephone company as such a fault, though, in fact, it is not.
Furthermore, because of objections from such telephone companies, the Federal Communications Commission has indicated an unwillingness to approve any Z class single line instrument, i.e. a single line instrument that has an on-hook impedance less than five megohm. Because of its bootstrap power supply, the line powered single line instrument of the Grandstaff patent is such an instrument. Thus, the line-powered product disclosed in the Grandstaff patent, while a technical advance, has not been widely adopted.
For the foregoing reasons, there is a need to provide an intrinsically safe telephone that does not draw line power in an on-hook condition.
One other limitation of the intrinsically safe telephone disclosed in the Grandstaff patent relates to its sidetone characteristics, i.e., it exhibits sidetones that are higher than those found in a typical telephone. The sidetone of a telephone is the acoustic signal resulting from a portion of the transmitted signal being coupled, within the telephone, to the receiver of the same handset. Some sidetone is required such that a caller may hear his own voice in the earpiece when speaking into the microphone of the telephone. If the amount of sidetone provided is too low, the caller has a tendency to unduly raise his voice when speaking. If the sidetone is too high, the caller speaks too softly. In a telephone designed for use in normal environments, there is an optimum amount of sidetone which is on the order of -12 db. In a telephone designed for use in noise environments, such as factories, mines and other locations in which the telephone of the Grandstaff patent is desirable for use, it is desired that a caller speak more loudly than in a normal environment. For this reason, in noisy environments even less than normal sidetone should be provided. In the noisy, hazardous environments in which an intrinsically safe telephone would be used, it would be desirable to provide between -12 and -14 db of sidetone.
The Grandstaff patent discloses an intrinsically safe telephone, however, in which the amount of sidetone provided is on the order of -5 db; an amount unsuitably high for environments with high ambient noise. Two conceivable methods of reducing the sidetone of the telephone of the Grandstaff patent would be to either employ additional passive, capacitive components in the hazardous environment, thus increasing the overall capacitance in the hazardous environment, or employing additional active components drawing current in the hazardous environment thus increasing the power required. The provision of additional capacitance or current drawing components in the single line instrument of the Grandstaff patent would cause it to be unsafe in a hazardous environment and thus, cease to be intrinsically safe. Another possible mechanism to reduce the sidetone would be to isolate the outgoing audio signal from the reflected sidetone signal by use of two separate two-wire paths from the hazardous to the safe environment. Such an approach, however, would also increase capacitance due to the presence of added wiring and would result in additional limitations regarding the maximum safe distance that the handset could be displaced from the safe environment.
For these latter reasons, there is a need to provide a telephone with reduced sidetone but which maintains intrinsic safety and without employment of two separate, physical two-wire paths.
Still another limitation that has arisen in practice with the intrinsically safe telephone disclosed in the Grandstaff patent relates to the off-hook detection circuit mentioned above. The telephone disclosed in the Grandstaff patent includes a telephone line interface located in a safe environment with a single line instrument located in the hazardous environment. The off-hook indicator circuit is provided in the telephone line interface and, in operation, that circuit detects a change in loop current which, in turn, indicates that the handset is off-hook. Because the single line instrument is located in a hazardous environment there are severe constraints regarding the permissible amplitude of the voltage input to the single line instrument and magnitude of current output from the single line instrument. Moreover, there is often a long distance between the telephone line interface and the single line instrument, reaching distances, for example, of up to one mile or more. For this reason, the line resistance may be quite high. Given these constraints, it is difficult to detect the change in loop current signifying an off-hook condition. This is especially so in the presence of incoming ringing signals which tend to mask the loop current change. With the telephone of the Grandstaff patent, a call recipient could take the phone off-hook in response to an incoming ring signal but that off-hook condition could remain undetected by the off-hook indicator circuit in the telephone line interface during an incoming ring signal. In this situation, when the call recipient placed the telephone receiver to the recipient's ear, the telephone would continue to ring. Such a condition, in practice, is undesirable.
Possible techniques for increasing the sensitivity of the off-hook detection capability of the Grandstaff telephone might be thought to include decreasing input ring voltage to or increasing output current from the single line instrument. Increasing the output current, however, would have the effect of increasing the spark potential of the single line instrument and thereby destroying its intrinsic safety. Decreasing the input ring voltage, on the other hand, would have the undesirable effect of making the ring signal less audible in a noisy environment.
Still another limitation of the Grandstaff telephone resides in the fact that the audible ring is sometimes not loud enough in particularly noisy environments such that call recipients are unable to detect the fact that the phone is ringing. A possible solution to such a condition would be to employ a louder ringing transducer. Such transducers, however, have increased capacitance to that employed in the Grandstaff patent. Such increased capacitance would destroy the intrinsic safety of the telephone.
From the foregoing it will be understood that there is a need to provide an intrinsically safe telephone having an increased off-hook detection capability without comprising its intrinsic safety.
Accordingly, it is an object of the present invention to provide an intrinsically safe telephone which is capable of operating in all hazardous environments as defined by the National Electrical Code, which does so without the use of an expensive explosion-proof housing, which meets all of the requirements of Underwriter's Laboratories for operation within hazardous environments and which is fully compatible with pre-existing telephone networks.
It is still a further object of the present invention to provide a louder audible ring without compromising intrinsic safety.
It is still further an object of the present invention to provide an intrinsically safe telephone in response to, and in satisfaction of, the aforementioned needs which does not draw line power in an on-hook condition, which has reduced sidetone, which has an increased off-hook detection capability, and which has a louder audible ring, all while maintaining, and without compromising, the intrinsic safety of the telephone.